1. Field of the Invention
The present invention relates to an optical head device for recording and reproducing information to/from an optical information recording medium.
2. Description of the Related Art
In order to meet the demand for a smaller optical disk apparatus for recording and reproducing information using laser light, it has conventionally been attempted to reduce the size and weight of the optical head. A typical example of such an apparatus is an optical disk apparatus for recording and reproducing information to/from a disk using a semiconductor laser. Such an apparatus requires a light beam to have a circular isointensity line shape in order to improve the light efficiency and/or to obtain a light spot with an axially-symmetrical intensity distribution. Hereinafter, in this specification, the isointensity line shape will be referred to as the "beam shape".
In general, when a light beam emitted from a semiconductor laser is collimated into collimated light with a collimator lens, the collimated light will have an elliptic beam shape. This is due to the difference between the radiation angle of the light beam along the P-N junction (i.e., the horizontal direction) of the semiconductor laser and that of the light beam along the direction vertical to the P-N junction of the semiconductor laser. Therefore, it has been conventionally proposed to provide an optical system for shaping the elliptic collimated light into circular collimated light.
FIG. 7 illustrates a structure of a conventional optical head device. A laser diode 23 as a laser light source emits a linearly-polarized light beam. The light beam emitted from the laser diode 23 is converted by a collimator lens 24 into collimated light having an elliptic beam shape. The laser diode 23 is located so that the long axis of the ellipse is substantially parallel to the thickness direction of the optical head device (i.e., the rotation axis of an optical information recording medium 31).
The collimated light having an elliptic beam shape is expanded in the horizontal direction by beam shaping prisms 25 and 26 so as to be shaped into a circular beam. The circular beam passes through a polarization beam splitter 27 and a .lambda./4 plate 28, and is then incident upon a deflection mirror 29. The polarization of the circular beam is converted by the .lambda./4 plate 28 from the linear polarization to the circular polarization. Thereafter, the circular beam is deflected by the deflection mirror 29 so as to direct the optical path thereof vertically toward the optical information recording medium 31. The beam is then converged by an objective lens 30 and is illuminated onto the optical information recording medium 31. The light converged by the objective lens 30 forms a tiny light spot on a surface of the optical information recording medium 31. Such a light spot enables the recording and erasing of information to/from the optical information recording medium 31.
The reflected light beam from the optical information recording medium 31 travels along a path in reverse to that of the light beam emitted from the laser diode 23, and passes through the .lambda./4 plate 28 again. The polarization of the light beam is converted by the .lambda./4 plate 28 from the circularly polarized to the linearly polarized. The polarization direction of the linearly-polarized light is perpendicular to the polarization direction of the light beam emitted from the laser diode 23.
The light beam having passed through the .lambda./4 plate 28 is reflected by the polarization beam splitter 27, thereby diverging from the optical path of the light beam emitted from the laser diode 23. Then, the light beam is converged by a detection lens 32 onto a signal detection prism 33.
Two light beams 35 and 36 partially reflected by the signal detection prism 33 and a light beam 37 totally reflected by the signal detection prism 33 are guided to a photodetector 34. The photodetector 34 is used to detect a focusing error signal FE, a tracking error signal TE and an information signal RF. For example, the focusing error signal FE is detected by the known spot size detection method (SSD), while the tracking error signal TE is detected by the known push-pull detection method. The information signal RF is detected based on the sum signal obtained from respective detected signals. The information signal RF represents a data signal recorded on the optical information recording medium 31.
FIG. 8 illustrates light receiving areas of the photodetector 34. The photodetector 34 has light receiving areas 34a to 34d for receiving circular beams. Circular light spots 38 and 39 are illuminated onto the light receiving areas 34d and 34c, respectively, while a circular light spot 40 is illuminated onto the light receiving areas 34a and 34b.
The light spots 38, 39 and 40 respectively correspond to the light beams 35, 36 and 37 resulting when the light spot on the optical information recording medium 31 is minimally small. The photodetector 34 is located between the respective focal points of the light beams 35 and 36 so that the light spot 38 is a near-field image pattern while the light spots 39 and 40 are each a far-field image pattern.
When signals detected by the light receiving areas 34a to 34d of the photodetector 34 are represented by S.sub.a to S.sub.d, respectively, the signals FE, TE and RF are expressed by Expressions (1) to (3) below, respectively. EQU FE=S.sub.c -S.sub.d (1) EQU TE=S.sub.a -S.sub.b (2) EQU RF=S.sub.a +S.sub.b +S.sub.c +S.sub.d (3)
In the above conventional example, the vertical thickness of the optical disk is limited by the size of the laser spot at the objective lens, whereby it is difficult to reduce the thickness of the optical head device. Moreover, in order to maintain the sensitivity of the focusing error signal FE, the distance between the detection lens 32 and the photodetector 34, as shown in FIG. 7, is limited, whereby it is difficult to reduce the size of the optical head device.